Living cells are capable of modulating the levels of proteins that they express. A variety of different mechanisms exist through which protein levels can be modulated. The ubiquitin pathway is one example of a post-translational mechanism used to regulate protein levels. Ubiquitin is a highly conserved polypeptide expressed in all eukaryotic cells that marks proteins for degradation. Ubiquitin is attached as a single molecule or as a conjugated form to lysine residue(s) of proteins via formation of an isopeptide bond at the C-terminal glycine residue. Most ubiquitinated proteins are subsequently targeted to the 26S proteasome, a multicatalytic protease, which cleaves the marked protein into peptide fragments.
Only the protein conjugated to ubiquitin is degraded via the proteasome; ubiquitin itself is recycled by ubiquitin carboxy-terminal hydrolases (UCH; sometimes abbreviated UCTH), which cleave the bond between ubiquitin and the protein targeted for degradation. These enzymes constitute a family of thiol proteases, and homologues have been found in, for example, yeast (Miller et al., BioTechnology 7:698-704, 1989; Tobias and Varshavsky, J. Biol. Chem. 266:12021-12028, 1991; Baker et al., J. Biol. Chem. 267:23364-23375, 1992), bovine (Papa and Hochstrasser, Nature 366:313-319, 1993), avian (Woo et al., J. Biol. Chem. 270:18766-18773, 1995), Drosophila (Zhang et al., Dev. Biol. 17:214, 1993) and human (Wilkinson et al., Science 246:670, 1989) cells.
Ubiquitination has been implicated in regulating numerous cellular processes including, for example, proliferation, differentiation, apoptosis (programmed cell death), transcription, signal-transduction, cell-cycle progression, receptor-mediated endocytosis, organelle biogenesis and others. The presence of abnormal amounts of ubiquitinated proteins in neuropathological conditions such as Alzheimer's and Pick's disease indicates that ubiquitination plays a role in various physiological disorders. Oncogenes (e.g., v-jun and v-fos) are often found to be resistant to ubiquitination in comparison to their normal cell counterparts, suggesting that a failure to degrade oncogene protein products accounts for some of their cell transformation capability. Combined with the observation that not all ubiquitinated proteins are degraded by the proteosome, these findings indicate that the process of ubiquitination and de-ubiquitination of particular substrates have important functional roles apart from recycling ubiquitin.